U.S. patent application number 11/843254 was filed with the patent office on 2008-10-23 for image forming apparatus.
This patent application is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Hiromitsu SHIMAZAKI.
Application Number | 20080260401 11/843254 |
Document ID | / |
Family ID | 39241398 |
Filed Date | 2008-10-23 |
United States Patent
Application |
20080260401 |
Kind Code |
A1 |
SHIMAZAKI; Hiromitsu |
October 23, 2008 |
IMAGE FORMING APPARATUS
Abstract
An image forming apparatus has a photo-conductor drum that bears
a toner image, and a transfer roller that is rotated in contact
with the photo-conductor drum. The transfer roller transfers the
toner image borne on the photo-conductor drum onto a recording
medium conveyed between the photo-conductor drum and the transfer
roller. The image forming apparatus includes a controller for
applying to the transfer roller a transfer bias voltage for
transferring the toner image from the photo-conductor drum onto the
recording medium. The transfer bias voltage is a convolution of DC
component and AC component.
Inventors: |
SHIMAZAKI; Hiromitsu;
(Fukuoka, JP) |
Correspondence
Address: |
GREENBLUM & BERNSTEIN, P.L.C.
1950 ROLAND CLARKE PLACE
RESTON
VA
20191
US
|
Assignee: |
Matsushita Electric Industrial Co.,
Ltd.
Osaka
JP
|
Family ID: |
39241398 |
Appl. No.: |
11/843254 |
Filed: |
August 22, 2007 |
Current U.S.
Class: |
399/66 |
Current CPC
Class: |
G03G 15/1675
20130101 |
Class at
Publication: |
399/66 |
International
Class: |
G03G 15/16 20060101
G03G015/16 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 2006 |
JP |
2006-235234 |
Claims
1. An image forming apparatus comprising: an image bearing member
configured to bear a toner image; a transfer roller configured to
be rotated in contact with said image bearing member, and to
transfer the toner image borne on the image bearing member onto a
recording medium, the recording medium being conveyed between the
image bearing member and the transfer roller; and controller
configured to apply, to the transfer roller, a transfer bias
voltage for transferring the toner image from the image bearing
member onto the recording medium, wherein said transfer bias
voltage is convolution of DC component and AC component.
2. The image forming apparatus according to claim 1, wherein when
the DC component is a voltage represented by a constant voltage
X(V), and the AC component is a voltage represented by a frequency
f(Hz) and an amplitude [P-P](V), the relations are the followings:
500V.ltoreq.|X(V)|.ltoreq.6000V 500 Hz.ltoreq.f(Hz).ltoreq.1000 Hz,
and [P-P](V).ltoreq.2|X(V)|
3. The image forming apparatus according to claim 2, wherein the
transfer roller is made of an elastic foam, in which the rubber
hardness of the transfer roller is from 10.degree. to 60.degree. in
Asker C.
4. The image forming apparatus according to claim 3, wherein the
nip width between said image bearing member and said transfer
roller is from 1 mm to 20 mm.
5. The image forming apparatus according to claim 1, wherein the
controller applies, to the transfer roller, a transfer bias voltage
that is convolution of DC component and AC component, when the
printing is performed on the back face in the double-sided
printing.
6. The image forming apparatus according to claim 1, wherein the
toner of positive polarity and the toner of negative polarity are
deposited on the image bearing member.
7. The image forming apparatus according to claim 6, wherein the
toner of positive polarity deposited from said image bearing member
onto the recording medium is restored to said image bearing member
by the transfer bias voltage that is convolution of DC component
and AC component.
8. The image forming apparatus according to claim 7, wherein the
toner of negative polarity remains on said recording medium.
Description
BACKGROUND
[0001] The present invention relates to an image forming apparatus
that provides an excellent print quality with less transfer
unevenness, white patch and fogging by controlling the voltage of a
transfer roller, and more particularly to an image forming
apparatus having a high print quality in the double-sided
printing.
[0002] In an image forming apparatus such as a copier, a printer, a
multi-functional machine or a facsimile apparatus using the
electrophotographic technology, a transfer device with a transfer
roller is widely used. This transfer device involves uniformly
charging a photo-conductor with a charger unit, scanning the
surface of this photo-conductor by a laser beam modulated with an
image signal to form an electrostatic latent image, transferring
and depositing the toner onto the photo-conductor on which the
electrostatic latent image is formed to make a visible image
(hereinafter a toner image), further conveying the recording medium
between the photo-conductor and the transfer roller while applying
a transfer bias voltage to the transfer roller, and transferring
the toner image on the photo-conductor onto the recording medium.
This transfer bias voltage is applied to transfer the toner image
onto the recording medium by supplying charges having opposite
polarity to the toner from the back side of the recording
medium.
[0003] Conventionally, to make the excellent transfer, it was
required to flow a current within a predetermined range in passing
the recording medium, whereby a control method such as constant
current control or constant voltage control was properly employed
according to the function of the device.
[0004] FIG. 6 is a constitutional view of the conventional transfer
device with constant voltage control.
[0005] In FIG. 6, reference numeral 101 denotes a photo-conductor
drum, reference numeral 102 denotes a transfer roller, reference
numeral 103 denotes a recording medium, reference numeral 104
denotes a non-magnetic one-component toner, and reference numeral
105 denotes a transfer bias control circuit. The toner 104 supplied
from a toner supply roller, not shown, is negatively charged
frictionally by a toner regulation blade, and transferred and
deposited onto the electrostatic latent image on the
photo-conductor drum 101 to visualize it. The recording medium is
passed in perfect timing for the photo-conductor drum 101, and the
back surface of the recording medium 103 is rendered in opposite
polarity by applying a positive DC voltage of the transfer bias
control circuit 105 to the transfer roller 102. Thereby, the toner
image of the photo-conductor drum 101 is transferred onto the
recording medium 103. Thereafter, the toner image is fixed by heat
and pressure of a heat roller and a pressure roller in a fixing
unit, not shown.
[0006] However, when the constant voltage control is performed, a
transfer failure may occur due to insufficient transfer current in
the low temperature and low humidity environment, if a voltage, for
example, 500V, suitable to achieve the excellent transfer under the
ordinary temperature and ordinary humidity is applied. Thus, if an
applied voltage is set to achieve the excellent transfer in the low
temperature and low humidity environment, fogging may occur in the
ordinary temperature and ordinary humidity environment, and the
high temperature and high humidity environment. To avoid this
contradiction, the constant current control is adapted, but there
is a problem that when the toner image is transferred onto the
recording medium having narrow width, a current density is
increased in a non-image area, and a voltage of the transfer roller
falls, causing a transfer failure due to another factor.
[0007] Therefore, the following image forming apparatus was offered
to achieve the stable transfer ability on the recording medium of
various sizes in various environments (refer to patent document 1).
This image forming apparatus performs the constant current control
for the transfer roller, if a nip part is in the non-image area
(beyond the size), to hold the voltage at this time, or performs
the constant voltage control at this held voltage if the nip part
is in the image area (within the size).
[0008] Similarly, a color image forming apparatus was offered in
which the constant voltage control is performed for the transfer
roller, and electrostatic separation is performed for a separation
brush with a separation bias voltage containing a DC component and
an AC component (refer to patent document 2). However, this is
limited to the case that the drum diameter of the photo-conductor
drum is 70 mm or greater, or the radius of curvature in the nip
part is 35 mm or greater.
[0009] [Patent document 1] JP-A-2-123385
[0010] [Patent document 2] JP-A-7-98548
[0011] The image forming apparatus of patent document 1 provided
the stable transfer ability with less fogging on the recording
media of various widths in various environments, but it was
required that the control method was changed depending on whether
the nip part is in the image area (within the size) or the
non-image area (beyond the size), whereby the control was complex
and the high print quality could not be achieved.
[0012] Also, the image forming apparatus of patent document 2
performs the constant voltage control for the transfer roller, and
makes the electrostatic separation by applying a separation bias
voltage to the separation brush, in which the consistency between
the transfer and the separation is achieved in a color inherent
process for superposing the toner image on the photo-conductor
drum. That is, this is the control specific to the color image
forming apparatus, and the control for the separation brush is
increased so that the overall control is more complex.
[0013] In this manner, the image forming apparatus of patent
document 1 or 2 can cope with the printing environment or the color
characteristics, but does not improve the print quality in
consideration of the toner property itself. Generally, the toner is
charged negatively or positively in a predetermined charged
polarity due to frictional charging with each charging member, but
because the disintegration or pulverization occurs due to collision
between toner particles during the agitation, such particles may be
charged in opposite polarity and mixed as the foreign matter into
the toner (some of the toner is charged in opposite polarity.) Fine
particles such as fractured toner or chip toner among such
particles cause the transfer unevenness, white patch or fogging.
FIG. 7 is an explanatory view showing agitation in the toner
bottle.
[0014] In FIG. 7, reference numeral 104a denotes a particle such as
positively (opposite polarity) charged toner, reference numeral
104b denotes a fine particle such as toner chip, reference numeral
106 denotes a toner bottle, and reference numeral 107 denotes a
toner agitation member. The toner agitation member 107 is rotated
in synchronism with the toner supply roller to negatively charge
the toner as well as prevent coagulation of the toner 104, to move
the toner to the toner supply roller 6. At this time, the toner 104
should be negatively charged uniformly, but some of the toner is
positively charged frictionally as the particle 104a of foreign
matter, or there is fine particle 104 as fractured toner or its
chip by the agitation.
[0015] Accordingly, if the toner image is formed by such toner 104,
the large positive particle 104a does not adhere to the
photo-conductor drum 101, but the fine particle 104b may adhere and
be transferred onto the recording medium. And the chip of fractured
toner 104 may float and adhere to other than the image area. This
is one of the causes for producing the white patch, transfer
unevenness and fogging, and degrading the print quality.
[0016] Also, in the double-sided printing, the printing is
performed on one side of the recording medium and then on the other
side, but the firstly printed image might degrade the print quality
when the printing is performed on the remaining face. The
conventional image forming apparatus of patent document 1 or 2 does
not solve the problem with the print quality that may arise in the
double-sided printing.
SUMMARY
[0017] Thus, it is an object of the invention to provide an image
forming apparatus that can form the favorable image having the
excellent print quality with less transfer unevenness, white patch
and fogging, and the high print quality in the double-sided
printing.
[0018] The present invention provides an image forming apparatus
that has an image bearing member and a transfer roller. The
photo-conductor drum bears a toner image. The transfer roller is
rotated in contact with the image bearing member, and transfers the
toner image borne on the image bearing member onto a recording
medium conveyed between the image bearing member and the transfer
roller. The image forming apparatus includes a controller for
applying to the transfer roller a transfer bias voltage for
transferring the toner image from the image bearing member onto the
recording medium. The transfer bias voltage is a convolution of DC
component and AC component.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a constitutional view of an image forming
apparatus according to an embodiment 1 of the present
invention.
[0020] FIG. 2 is an explanatory view showing the constitution of a
main part of the image forming apparatus according to the
embodiment 1 of the invention.
[0021] FIG. 3A is an explanatory view for explaining the operation
when a transfer bias voltage that is convolution of DC component
and AC component is applied to the image forming apparatus
according to the embodiment 1 of the invention and FIG. 3B is an
explanatory view for explaining the operation when a transfer bias
voltage of DC component is applied to the image forming apparatus
of FIG. 3A.
[0022] FIG. 4 is a view showing the relationship between the rubber
hardness and the transfer efficiency of the transfer roller.
[0023] FIG. 5A is an explanatory view for explaining the operation
when a transfer bias voltage that is convolution of DC component
and AC component is applied to an image forming apparatus according
to an embodiment 2 of the invention and FIG. 5B is an explanatory
view for explaining the operation when a transfer bias voltage of
DC component is applied to the image forming apparatus of FIG.
5A.
[0024] FIG. 6 is a constitutional view of the conventional transfer
device under the constant voltage control.
[0025] FIG. 7 is an explanatory view showing how the agitation is
made in the toner bottle.
DETAILED DESCRIPTION
Embodiment 1
[0026] An image forming apparatus according to an embodiment 1 of
the present invention will be described below in detail with
reference to the accompanying drawings. The image forming apparatus
of the invention forms the image through an electrophotographic
process, or transfers the image formed using the toner, such as a
printer, a copier or a multi-functional machine.
[0027] FIG. 1 is a constitutional view of an image forming
apparatus according to an embodiment 1 of the invention. FIG. 2 is
an explanatory view showing the constitution of a main part of the
image forming apparatus according to the embodiment 1 of the
invention. FIG. 3A is an explanatory view for explaining the
operation when a transfer bias voltage that is convolution of DC
component and AC component is applied to the image forming
apparatus according to the embodiment 1 of the invention, and FIG.
3B is an explanatory view for explaining the operation when a
transfer bias voltage of DC component is applied to the image
forming apparatus of FIG. 3A. FIG. 4 is a view showing the
relationship between the rubber hardness and the transfer
efficiency of the transfer roller.
[0028] The image forming apparatus of the embodiment 1 can perform
the single-sided printing, but may not perform the double-sided
printing. In the following, the image forming apparatus capable of
double-sided printing will be described in the case of single-sided
printing. The details of the image forming apparatus in the
double-sided printing will be described later in an embodiment 2.
In the embodiment 1, the single-sided printing is treated. Herein,
a conveying path A of the image forming apparatus as shown in FIG.
1 conveys the recording medium such as the copy paper in the
single-sided printing, and a conveying path B is used for the
double-sided printing to reverse and insert the recording medium
outputted onto a paper output tray in the double-sided
printing.
[0029] In FIG. 1, reference numeral 1 denotes a photo-conductor
drum (image bearing member of the invention) for forming an
electrostatic latent image, in which a photosensitive layer is
formed on the outer peripheral surface of a metallic drum made of
aluminum or the like. Reference numeral 2 denotes a charger unit,
which is disposed near the photo-conductor drum 1, for uniformly
charging the photo-conductor drum 1 by corona discharge. Reference
numeral 3 denotes exposure means such as a laser diode, and
reference numeral 3a denotes a light beam such as a laser beam
radiated from the exposure means 3. The exposure means 3 generates
a light beam 3a by modulating an image signal in a drive circuit of
the exposure means 3, based on the image signal acquired by an
image sensor, or the image signal such as recorded data, and scans
the surface of the photo-conductor drum 1 by this light beam to
form an electrostatic latent image.
[0030] Reference numeral 4 denotes a static eliminator for
eliminating the static electricity by applying the light, and
reference numeral 5 denotes a cleaning blade for scraping off the
residual toner remaining on the surface of the photo-conductor drum
1. Also, reference numeral 6 denotes a developing roller that is a
toner carrier, and reference numeral 7 denotes a toner supply
roller for supplying the negatively charged toner 9 onto the
surface of the developing roller 6, in which the developing roller
6 and the toner supply roller 7 are provided in contact. A toner
supply bias voltage that is convolution of DC component and AC
component is applied to the toner supply roller 7 by toner supply
bias voltage control means 22 (hereinafter described), and a
developing bias voltage of constant DC voltage is applied to the
developing roller 6 by developing bias voltage control means 23
(hereinafter described).
[0031] Further, reference numeral 8 denotes a toner bottle,
reference numeral 9 denotes a toner, and reference numeral 10
denotes a toner agitation member for agitating the toner 9. Also,
reference numeral 11 denotes a toner regulation blade, and
reference numeral 12 denotes a developer unit for storing the toner
bottle 8, the developing roller 6 and the toner supply roller 7.
The toner 9 of the embodiment 1 is a non-magnetic, one-component
toner, in which carbon, wax and a static control agent are
uniformly dispersed in polyester resin. The toner is agitated by
the toner agitation member 10 and negatively changed.
[0032] Also, the developing roller 6 is supported rotationally at
both ends of the developer unit 12, and formed with a conductive
elastic member, for example, a silicon rubber layer around the
outer periphery of a metallic shaft. The metallic shaft of the
developing roller 6 is connected to a constant voltage power
supply. Similarly, the toner supply roller 7 is also formed with a
conductive elastic foam around the periphery of a metallic shaft,
which is connected to a toner supply bias power supply. This toner
supply bias voltage is convolution of DC component and AC
component, whereby the toner supply roller 7 can supply the toner 9
supplied from the developer unit 12 to the developing roller 6, and
the toner 9 on the developing roller 6 that remains without being
developed can be effectively scraped off.
[0033] The toner 9 supplied by the toner supply roller 7 forms a
thin toner layer on the outer peripheral face of the developing
roller 6 owing to the toner regulation blade 11. The toner
regulation blade 11 is composed of a metallic plate spring and an
elastic member of urethane rubber or silicone rubber at one end
thereof, and presses the developing roller 6 at a line pressure of
about 80 g/cm to form a toner layer on the surface of the
developing roller 6. This developing roller 6 is disposed in
contact with the photo-conductor drum 1, whereby the toner 9 on the
toner layer is transferred and deposited on a part where the
electrostatic latent image is formed on the photo-conductor drum 1
by a bias voltage applied by the developing bias voltage means 22
to visualize the electrostatic latent image as a toner image.
[0034] By the way, the transfer device of the embodiment 1 will be
described below. In FIG. 1, reference numeral 13 denotes a transfer
roller rotating in contact with the photo-conductor drum 1. The
transfer roller 13 is formed with a conductive elastic foam, viz.,
a silicon rubber layer having a rubber hardness of 10.degree. to
60.degree. in Asker C in the embodiment 1, around the outer
periphery of a metallic shaft made of stainless or the like. A
positive voltage having the opposite polarity to the toner 9, which
is convolution of DC component and AC component, is applied to the
metallic shaft of this transfer roller 13 by the transfer bias
voltage control means 24, so that the toner image on the
photo-conductor drum 1 is transferred onto the recording medium.
The details thereof will be described later.
[0035] In the embodiment 1, with the constitution of this transfer
roller 13 as described above, the nip width between the
photo-conductor drum 1 and the transfer roller 13 is made from 1 mm
to 20 mm. With this setting, a variation in the electric field due
to AC component prevents the positively charged fine particle 9a
such as fractured toner 9 or its chip from adhering to the
photo-conductor drum 1 and the image area on the recording medium,
and restores the fractured toner or its chip to the photo-conductor
drum 1, even if the fine particle floats and adheres to the
non-image area on the recording medium, whereby it is possible to
realize the suitable relationship between the photo-conductor drum
1 and the transfer roller 13, and improve the print quality by
preventing the white patch, unevenness of printing, and fogging.
The specific control therefor will be described later.
[0036] In FIG. 1, reference numeral 14 denotes a fixing device,
reference numeral 15 denotes a heat roller that is internally
provided with a heat source such as a halogen lamp, and reference
numeral 16 denotes a pressure roller pushed against the heat roller
15 and formed with a rubber layer or a sponge layer on the surface.
The pressure roller 16 is pushed against the heat roller 15 by a
spring or the like.
[0037] Also, reference numeral 17 denotes a paper feed tray that
stacks the recording media such as paper, reference numeral 18
denotes a pickup roller for picking up the recording medium with a
partition plate and carrying in the recording medium one by one
from the paper feed tray 17 onto the conveying path A, and
reference numeral 19 denotes a conveying roller. And reference
numeral 19a denotes a registration roller for temporarily stopping
the recording medium in standby to align the recording medium with
the toner image on the photo-conductor drum 1, the registration
roller being contacted with a slave roller. Also, reference numeral
20 denotes a paper output tray, and reference numeral 21 denotes a
double-sided printing conveying roller for conveying the recording
medium onto the conveying path B when the recording medium is
inserted through an exhaust port of the paper output tray 20 for
the second printing in the double-sided printing.
[0038] And reference numeral 22 denotes toner supply bias voltage
control means for applying a toner supply bias voltage that is
convolution of DC component and AC component to the toner supply
roller 7. Also, reference numeral 23 denotes developing bias
voltage control means for applying a developing bias voltage that
is a constant DC voltage to the developing roller 6. The developing
bias voltage control means 23 and the toner supply bias voltage
control means 22 cooperate to make the control.
[0039] The developing bias voltage control means 23 controls the
developing bias voltage to be at least 100V or greater, and the
toner supply bias voltage control means controls the DC component
voltage of the toner supply bias voltage to be greater than or
equal to this developing bias voltage in accordance with the
control of this developing bias voltage. Further, the toner supply
bias voltage control means 22 controls the peak-to-peak amplitude
(hereinafter amplitude [P-P](V)) of the AC component of the toner
supply bias voltage to be smaller than the DC component, and
slightly greater than a potential difference in the DC component
between the developing bias voltage and the toner supply bias
voltage. Under the cooperative control of the developing bias
voltage control means 23 and the toner supply bias voltage control
means 22, it is possible to effectively convey the toner to the
developing roller 6 and scrape off the unconsumed toner. The toner
supply roller 7 may be grounded, or have any other control means
for applying only the DC voltage, whereby the same effect is
obtained.
[0040] In FIG. 1, reference numeral 24 denotes transfer bias
voltage control means that is a feature of the embodiment 1. The
transfer bias voltage control means 24 applies a voltage having the
opposite polarity (positive) to the toner 9, which is convolution
of DC component and AC component, to transfer the toner image on
the photo-conductor drum 1. The details thereof will be described
below with reference to FIG. 2 and FIGS. 3A and 3B. In FIG. 2,
reference numeral 9a denotes fine particle having the opposite
polarity (positive) to the toner 9, reference numeral 24a denotes a
DC power supply controlled by the transfer bias voltage control
means 24, and reference numeral 24b denotes an AC power supply
controlled by the transfer bias voltage control means 24. Also,
reference numeral 25 denotes the recording medium such as
paper.
[0041] In the embodiment 1, the polarity of the toner 9 is
negative, and the polarity of the photo-conductor drum 1 and the
transfer roller 13 is positive. Also, the polarity of fine particle
9a is positive. The toner 9 is deposited on the photo-conductor
drum 1 to form an electrostatic latent image that is transferred
onto the recording medium 25, but the particles having large mass
(such as toner charged in opposite polarity) are not deposited, or
rarely deposited, on the photo-conductor drum 1 and the recording
medium 25. However, the fine particles 9a such as the fractured
toner 9 or its chip are subjected to a physical force other than an
electrostatic force such as Van der Waals force, and deposited on
the photo-conductor drum 1 and transferred onto the recording
medium 25. In the embodiment 1, the center particle diameter of the
toner 9 is roughly from 10 .mu.m to 12 .mu.m, and the size of fine
particles 9a is much smaller.
[0042] In FIG. 3B, a DC transfer bias voltage is applied
conventionally. A predetermined uniform electric field having less
variation from the transfer roller 13 to the photo-conductor drum 1
is applied to the toner 9 and fine particles 9a deposited on the
photo-conductor drum 1, so that the toner 9 is transferred onto the
recording medium. At this time, the physical force, in addition to
the electric force, is applied to the fine particles 9a having
opposite polarity, so that this physical force overcomes the
electrostatic force to cause some of the fine particles 9a to be
transferred and deposited on the recording medium.
[0043] However, when the transfer bias voltage control means 24
applies a transfer bias voltage that is convolution of DC component
and AC component, as shown in FIG. 3A, electric charges migrate in
both the front and back directions (bi-direction) across the
recording medium 25 due to polarization occurring on the front and
back surfaces of the recording medium 25, causing the electric
field between the transfer roller 13 and the photo-conductor drum 1
to be varied, and when the electric field in the reverse direction
is formed due to the AC component, the electrostatic force acts as
a resistive force in the reserve direction against the physical
force acting on the fine particles 9a having opposite polarity.
[0044] Therefore, the fine particles 9a of the fractured toner 9 or
its chip may not be dropped away from the photo-conductor drum 1,
or can be restored even if they are dropped away. Thereby, the fine
particles 9a can be prevented from being transferred onto the
recording medium 25. Also, the fine particles 9a are prevented from
floating and adhering to the non-image area to cause fogging, after
they are dropped away from the photo-conductor drum 1. However, in
the embodiment 1, the transfer bias voltage itself is totally a
positive value though the electric field is changed by the AC
component, whereby an electric force toward the transfer roller 13
can be applied to the toner 9. Accordingly, the white patch,
transfer unevenness or fogging does not occur to degrade the print
quality and lower the transfer efficiency, although they occurred
conventionally.
[0045] By the way, the image forming apparatus of the embodiment 1
as described above operates as follows. The pickup roller 18 picks
up the recording medium one by one from the paper feed tray 17 and
carries in the recording medium onto the conveying path A, so that
the recording medium is conveyed to the position of the
registration roller 19a by the conveying rollers 19. When the toner
image reaches a contact part between the transfer roller 13 and the
photo-conductor drum 1 along with the rotation of the
photo-conductor drum 1, the recording medium is controlled to reach
this contact part in perfect timing for the toner image by the
registration roller 19a. A transfer bias voltage having opposite
polarity to the toner 9 is applied to the transfer roller 13, so
that the toner image on the photo-conductor drum 1 is transferred
onto the recording medium due to an electrostatic force and so on.
Thereafter, the recording medium is conveyed to the fixing device
14, where the toner image is fixed by heat and pressure of the heat
roller 15 and the pressure roller 16, and exhausted onto the paper
output tray 20.
[0046] Thus, to examine the particulars of the control of the
transfer bias voltage that is convolution of DC component and AC
component, the print quality was evaluated by changing the
developing bias voltage X(V), the AC component frequency f(Hz) and
the amplitude [P-P](V). At first, a suitable range of rubber
hardness of the transfer roller is obtained from the viewpoint of
the transfer efficiency, and the developing bias voltage X(V), the
AC component frequency f(Hz) and the amplitude [P-P](V) were
changed with this rubber hardness as a parameter. The nip width is
suitably from 1 mm to 20 mm, as described above, but fixed at 3 mm
in this embodiment 1. And the print quality was evaluated based on
the occurrence situation of fogging, image quality such as white
patch and transfer unevenness, and contamination of the transfer
roller. The evaluation was performed visually. The evaluation was
given at three grades of A (good), B (acceptable), and C (no good).
The temperature and humidity were in the ordinary temperature (room
temperature) and ordinary humidity environment.
[0047] In FIG. 4, the rubber hardness of the transfer roller is
Asker C. The single-sided printing was performed at a transfer bias
voltage of 1 KV, in which the frequency f(Hz) was 1000 Hz and the
AC component amplitude [P-P](V) was 500V. In FIG. 4, the transfer
efficiency rises rapidly from the position of the rubber hardness
of 10.degree. and reaches 93% at 60.degree.. And the transfer
efficiency is kept at 93% to 94% up to near the rubber hardness of
60.degree., indicates about 91% at the rubber hardness of
70.degree., and indicates 85% near the rubber hardness of
75.degree.. Table 1 lists the results of this experiment. According
to FIG. 4 and Table 1, the rubber hardness of the transfer roller
is suitably from 10.degree. to 60.degree. in Asker C, based on the
evaluation of this experiment, to improve the print quality.
TABLE-US-00001 TABLE 1 Transfer NIP Transfer voltage: X Frequency:
f Amplitude: width efficiency Image Contamination of (V) (Hz) P-P
(V) (mm) (%) Fogging quality transfer roller 300 100 500 3 70 B B B
400 100 500 3 72 B B B 500 500 500 3 93 A A A 1000 500 500 3 94 A A
A 1500 500 500 3 94 A A A 2000 1000 500 3 94 A A A 2500 1000 500 3
94 A A A 3000 1000 500 3 93 A A A 3500 1000 500 3 94 A A A 4000
1000 500 3 94 A A A 4500 1000 500 3 92 A A A 5000 1000 500 3 92 A A
A 5500 1000 500 3 90 A A A 6000 1000 500 3 90 A A A 6500 2000 500 3
83 B B B 7000 2000 500 3 80 B B B 7500 2000 500 3 70 B B B
[0048] Next, when the developing bias voltage X(V), the AC
component frequency f(Hz) and the amplitude [P-P](V) are changed
with the rubber hardness of the transfer efficiency as a parameter,
the results are shown below. This is also the case of single-sided
printing. Table 2 lists the results where the rubber hardness of
the transfer roller is 20.degree. in Asker C.
TABLE-US-00002 TABLE 2 Transfer NIP Transfer voltage: X Frequency:
f Amplitude: width efficiency Image Contamination of (V) (Hz) P-P
(V) (mm) (%) Fogging quality transfer roller 300 100 500 3 70 B B B
400 100 500 3 70 B B B 500 500 500 3 90 A A A 1000 500 500 3 91 A A
A 1500 500 500 3 90 A A A 2000 1000 500 3 90 A A A 2500 1000 500 3
90 A A A 3000 1000 500 3 91 A A A 3500 1000 500 3 91 A A A 4000
1000 500 3 91 A A A 4500 1000 500 3 90 A A A 5000 1000 500 3 89 A A
A 5500 1000 500 3 89 A A A 6000 1000 500 3 88 A A A 6500 2000 500 3
82 B B B 7000 2000 500 3 80 B B B 7500 2000 500 3 65 B C C
Also, Table 3 lists the results where the rubber hardness of the
transfer roller is 400 in Asker C. This is also the case of
single-sided printing.
TABLE-US-00003 TABLE 3 Transfer NIP Transfer voltage: X Frequency:
f Amplitude: width efficiency Image Contamination of (V) (Hz) P-P
(V) (mm) (%) Fogging quality transfer roller 300 100 500 3 75 B B B
400 100 500 3 76 B B B 500 500 500 3 92 A A A 1000 500 500 3 93 A A
A 1500 500 500 3 93 A A A 2000 1000 500 3 93 A A A 2500 1000 500 3
94 A A A 3000 1000 500 3 93 A A A 3500 1000 500 3 94 A A A 4000
1000 500 3 94 A A A 4500 1000 500 3 93 A A A 5000 1000 500 3 94 A A
A 5500 1000 500 3 94 A A A 6000 1000 500 3 93 A A A 6500 2000 500 3
85 B B B 7000 2000 500 3 80 B B B 7500 2000 500 3 76 B B B
[0049] Further, Table 4 lists the results where the rubber hardness
of the transfer roller is 60.degree. in Asker C. This is the case
of single-sided printing.
TABLE-US-00004 TABLE 4 Transfer NIP Transfer voltage: X Frequency:
f Amplitude: width efficiency Image Contamination of (V) (Hz) P-P
(V) (mm) (%) Fogging quality transfer roller 300 100 500 3 73 B B B
400 100 500 3 74 B B B 500 500 500 3 90 A A A 1000 500 500 3 92 A A
A 1500 500 500 3 92 A A A 2000 1000 500 3 91 A A A 2500 1000 500 3
91 A A A 3000 1000 500 3 92 A A A 3500 1000 500 3 92 A A A 4000
1000 500 3 93 A A A 4500 1000 500 3 90 A A A 5000 1000 500 3 92 A A
A 5500 1000 500 3 91 A A A 6000 1000 500 3 90 A A A 6500 2000 500 3
85 B B B 7000 2000 500 3 80 B B B 7500 2000 500 3 76 B B B
[0050] According to Tables 2 to 4, when the transfer voltage X(V)
is from 500V to 6000V, the frequency f(Hz) is from 500 Hz to 1000
Hz, and the amplitude [P-P](V) is 500V, the transfer efficiency is
as high as about 90%, and the fogging, white patch or transfer
unevenness, and the contamination of the transfer roller are small.
If the amplitude [P-P](V) exceeds double the absolute value of
transfer voltage X(V), the electric field between the transfer
roller 13 and the photo-conductor drum 1 is changed in the
direction and varied to impede the transfer of the negatively
charged toner 9 itself, and can not be employed. Accordingly, the
amplitude [P-P](V).ltoreq.2|X(V)| is a condition for not degrading
the print quality. Herein, |X(V)| indicates the absolute value of
X(V).
[0051] The image forming apparatus of the embodiment 1 does not set
the conditions minutely according to an environmental change and
properly use the constant current control and the constant voltage
control to keep the print quality, as conventionally, but applies a
convolution of AC component and DC component to secure the print
quality owing to its relation and the action of AC component.
Therefore, the control is facilitated, and the print quality can be
securely improved. Though not displayed here, even when the
frequency f(Hz) is changed in a wider range from 50 Hz to 5000 Hz,
the image forming apparatus can be provided in which the fogging,
white patch and transfer unevenness are relatively small and the
contamination of the transfer roller is small by choosing the
transfer voltage X(V) and the amplitude [P-P](V).
[0052] In this manner, the image forming apparatus of the
embodiment 1 can provide the excellent print quality in which the
transfer unevenness, white patch and fogging are small because the
transfer bias voltage is convolution of DC component and AC
component. Also, when the DC component is constant voltage X(V),
and the AC component has frequency f(Hz) and the amplitude
[P-P](V), the transfer unevenness, white patch and fogging can be
securely prevented under the conditions where
500V.ltoreq.|X(V)|.ltoreq.6000V, 500 Hz.ltoreq.f(Hz).ltoreq.1000
Hz, and [P-P](V).ltoreq.2|X|.
Embodiment 2
[0053] Next, an image forming apparatus according to an embodiment
2 of the present invention, and more particularly an image forming
apparatus capable of double-sided printing, will be described below
in detail. While the single-sided printing of the image forming
apparatus capable of double-sided printing has been described in
the embodiment 1, the double-sided printing will be described in
the embodiment 2. Accordingly, the image forming apparatuses of the
embodiments 1 and 2 have the same constitution, in which the same
parts are denoted by the same reference numerals, and not described
here.
[0054] The image forming apparatus of the embodiment 2 can perform
the double-sided printing in which the initial (first) printing is
made on the conveying path A as shown in FIG. 1, the recording
medium exhausted onto the paper output tray is reversed and
inserted, and then conveyed on the conveying path B, and the second
printing is made on the conveying path A again.
[0055] FIG. 5A is an explanatory view for explaining the operation
when a transfer bias voltage that is convolution of DC component
and AC component is applied to the image forming apparatus
according to the embodiment 2 of the invention, and FIG. 5B is an
explanatory view for explaining the operation when a transfer bias
voltage of DC component is applied to the image forming apparatus
of FIG. 5A.
[0056] By the way, when the double-sided printing is performed, the
side of the recording medium on which the image is fixed by the
first printing is reversed, and the toner image is transferred.
Accordingly, the recording medium 25 to be conveyed has the toner 9
fixed by the first printing on the back surface, in which the toner
9 (image area) fixed by the first transfer may change the electric
field for the transfer in the second printing.
[0057] That is, the dielectric constant is lower due to the fixed
toner 9 in the image area, and the polarizability of the recording
medium 25 is lower, so that the electric field strength between the
transfer roller 13 and the photo-conductor drum 1 falls.
Accordingly, when only the DC voltage is applied, the electric
field is insufficient in the image area where the toner is fixed in
the first printing, so that the toner 9 to be moved and deposited
is not transferred, and the small fine particles 9a such as
fractured toner 9 or its chip may adhere, as shown in FIG. 5B. In
FIG. 5B, reference numeral 9b denotes the toner to be essentially
transferred, but actually not transferred. Also, reference numeral
26 denotes the image area where the toner 9 is fixed in the first
printing. The other area than the image area is the non-image
area.
[0058] However, in the embodiment 2, when the transfer bias voltage
control means 24 applies a transfer bias voltage that is
convolution of DC component and AC component, as shown in FIG. 5A,
the electric field between the transfer roller 13 and the
photo-conductor drum 1 is varied alternately, whether the image
area or the non-image area, even if the polarizability of the
recording medium 25 falls. The electric fields for the image area
and the non-image area in the first printing are averaged, whereby
there is a smaller difference in the electric field between both on
average, so that the transfer efficiency of the already printed
image area part is not lower than the transfer efficiency of the
non-image area part. The state of the first printing ranges from
duty 0% to 100%, whereby even if any printing density occurs on the
first printing face, the transfer unevenness is averaged by
combining the AC component at the second time of transfer, so that
the smooth transfer can be achieved without problem.
[0059] Thus, in the embodiment 2, the print quality was evaluated
by changing the developing bias voltage X(V), the AC component
frequency f(Hz) and the amplitude [P-P](V). The rubber hardness of
the transfer roller was changed as a parameter. The nip width was 3
mm. The print quality was evaluated based on the occurrence
situation of fogging, image quality of white patch and transfer
unevenness, and contamination of the transfer roller, like the
embodiment 1. The evaluation was performed visually. The evaluation
was given at three stages of O, .DELTA. and x. Table 5 lists the
results in which the rubber hardness of the transfer roller is
20.degree. in Asker C in the double-sided printing.
TABLE-US-00005 TABLE 5 Transfer NIP Transfer voltage: X Frequency:
f Amplitude: width efficiency Image Contamination of (V) (Hz) P-P
(V) (mm) (%) Fogging quality transfer roller 300 100 500 3 68 C B C
400 100 500 3 74 B B B 500 500 500 3 89 A A A 1000 500 500 3 91 A A
A 1500 500 500 3 93 A A A 2000 1000 500 3 92 A A A 2500 1000 500 3
93 A A A 3000 1000 500 3 93 A A A 3500 1000 500 3 93 A A A 4000
1000 500 3 93 A A A 4500 1000 500 3 93 A A A 5000 1000 500 3 92 A A
A 5500 1000 500 3 91 A A A 6000 1000 500 3 91 A A A 6500 2000 500 3
80 B B B 7000 2000 500 3 76 C C B 7500 2000 500 3 70 C C C
[0060] Also, Table 6 lists the results in which the rubber hardness
of the transfer roller is 40.degree. in Asker C in the double-sided
printing.
TABLE-US-00006 TABLE 6 Transfer NIP Transfer voltage: X Frequency:
f Amplitude: width efficiency Image Contamination of (V) (Hz) P-P
(V) (mm) (%) Fogging quality transfer roller 300 100 500 3 67 C B C
400 100 500 3 72 B C B 500 500 500 3 90 A A A 1000 500 500 3 90 A A
A 1500 500 500 3 90 A A A 2000 1000 500 3 90 A A A 2500 1000 500 3
90 A A A 3000 1000 500 3 91 A A A 3500 1000 500 3 90 A A A 4000
1000 500 3 90 A A A 4500 1000 500 3 88 A A A 5000 1000 500 3 89 A A
A 5500 1000 500 3 90 A A A 6000 1000 500 3 88 A A A 6500 2000 500 3
82 B B B 7000 2000 500 3 78 C C B 7500 2000 500 3 65 C C C
[0061] Further, Table 7 lists the results in which the rubber
hardness of the transfer roller is 60.degree. in Asker C in the
double-sided printing.
TABLE-US-00007 TABLE 7 Rubber NIP Transfer hardness Frequency: f
Amplitude: width efficiency Image Contamination of (Asker.degree.)
(Hz) P-P (V) (mm) (%) Fogging quality transfer roller 5 1000 500 3
60 B B B 8 1000 500 3 65 B B B 9 1000 500 3 70 A A A 10 1000 500 3
93 A A A 15 1000 500 3 94 A A A 20 1000 500 3 94 A A A 30 1000 500
3 94 A A A 40 1000 500 3 94 A A A 50 1000 500 3 94 A A A 60 1000
500 3 93 A A A 65 1000 500 3 92 A A A 70 1000 500 3 91 A A A 75
1000 500 3 85 A A B
[0062] According to Tables 5 to 7, like the embodiment 1, when the
transfer voltage X(V) is from 500V to 6000V, the frequency f(Hz) is
from 500 Hz to 1000 Hz, and the amplitude [P-P](V) is 500V, the
transfer efficiency is as high as about 90% or more, and the
fogging, white patch or transfer unevenness, and the contamination
of the transfer roller are small. If the amplitude [P-P](V) exceeds
double the absolute value of the transfer voltage X(V), the
electric field between the transfer roller 13 and the
photo-conductor drum 1 is changed in the direction and varied to
impede the transfer of the negatively charged toner 9 itself, and
can not be employed. Accordingly, the amplitude
[P-P](V).ltoreq.2|X(V)| is a condition for not degrading the print
quality.
[0063] The image forming apparatus of the embodiment 2, like the
embodiment 1, does not set the conditions minutely according to an
environmental change and properly use the constant current control
and the constant voltage control to keep the print quality, as
conventionally, but applies a convolution of AC component and DC
component to secure the print quality owing to its relation and the
action of AC component. Therefore, the control is facilitated, and
the print quality can be securely improved. And even when the
frequency f(Hz) is changed from 50 Hz to 5000 Hz, the image forming
apparatus can be provided in which the fogging, white patch and
transfer unevenness are relatively small and the contamination of
the transfer roller is small by choosing the transfer voltage X(V)
and the amplitude [P-P](V).
[0064] The image forming apparatus of the embodiment 2 as described
above operates in the double-sided printing as follows. The pickup
roller 18 picks up the recording medium 25 one by one from the
paper feed tray 17, and carries in the recording medium onto the
conveying path A, so that the recording medium is conveyed to the
position of the registration roller 19a by the conveying rollers
19. When the toner image reaches a contact part between the
transfer roller 13 and the photo-conductor drum 1 along with the
rotation of the photo-conductor drum 1, the recording medium 25 is
controlled to reach this contact part in perfect timing for the
toner image by the registration roller 19a. A transfer bias voltage
having opposite polarity to the toner 9 is applied to the transfer
roller 13, so that the toner image on the photo-conductor drum 1 is
transferred onto the recording medium 25 due to an electrostatic
force and so on. Thereafter, the recording medium 25 is conveyed to
the fixing device 14, where the toner image is fixed by heat and
pressure of the heat roller 15 and the pressure roller 16, and
exhausted onto the paper output tray 20.
[0065] Thereafter, because of the double-sided printing, the
recording medium is inserted manually or automatically through the
exhaust opening of the paper output tray 20 for the second
printing. At this time, the conveying roller 19 is controlled to be
reversely rotated, whereby if the recording medium reaches the
position of the conveying roller 21 for double-sided printing, the
recording medium 25 is conveyed on the conveying path B due to a
rotational force of the conveying roller 21 for double-sided
printing. The recording medium is conveyed via the conveying path B
to the position of the registration roller 19a again, so that the
toner image on the photo-conductor drum 1 is transferred onto the
back surface of the recording medium 25. Thereafter, the recording
medium 25 is fixed in the fixing device 14, and outputted onto the
paper output tray 20.
[0066] Incidentally, the image forming apparatus of the embodiment
2 has been described above, using mainly the monochrome image
forming apparatus. The invention may be also applicable to a color
image forming apparatus comprising the developer unit 12 having the
toner bottles of yellow (Y), magenta (M), cyan (C) and black (K),
which performs the printing repeatedly plural times in such a
manner that the first printing is performed in yellow, the second
printing is performed in magenta, third printing is performed in
cyan and the fourth printing is performed in black. That is, in the
repetitive transfer of four colors, the print quality may be
degraded unless the electrostatic separation is securely
controlled, but the image forming apparatus of the embodiment 2 can
avoid this problem simply, and the color image forming apparatus is
usable for the single-sided printing and double-sided printing.
Especially in the double-sided printing of the color image forming
apparatus, the print quality can be improved, unlike the
conventional image forming apparatus.
[0067] In this manner, the image forming apparatus of the
embodiment 2 can provide the excellent print quality in which the
transfer unevenness, white patch and fogging that are more likely
to occur in the double-sided printing are small, because the
transfer bias voltage is convolution of DC component and AC
component. Also, when the DC component is X(V), the AC component
has frequency is f(Hz), and the amplitude is [P-P](V), the transfer
unevenness, white patch and fogging can be securely prevented under
the conditions where 500V.ltoreq.|X(V)|.ltoreq.6000V, 500
Hz.ltoreq.f(Hz).ltoreq.1000 Hz, and [P-P](V).ltoreq.2|X|.
* * * * *